The present invention relates to a pulse width modulation circuit and a voltage-feedback class-D amplifier circuit using the same, especially, relates to configuration for improving the frequency characteristic of a circuit part which performs pulse width modulation by comparing an input signal with a triangular wave.
In AV (audio/visual) equipment etc., a portion for driving a load performs internal signal processing in digital, and an amplifier is employed when a load such as a motor and an actuator is driven by generating an output signal corresponding to an input signal. Generally, a digital amplifier is used for such an amplifier.
The amplification system of the digital amplifier is a class D amplifier system. In the digital amplifier, an input signal is compared in level with a triangular wave generated inside, and a signal is generated according to the comparison result; accordingly, the input signal is transformed into a pulse-width-modulated signal (PWM signal). A switching transistor of an output stage is driven according to the PWM signal, and the switching transistor of the output stage will be in ON and OFF states (conduction and non-conduction states) for a period corresponding to the pulse width. Accordingly, a signal corresponding to the input signal is generated at the output. The digital amplifier (class D amplifier) carries out only switching operation of the output stage transistor between ON and OFF states; therefore, the digital amplifier is excellent in power efficiency.
In the class D amplifier (digital amplifier), an output signal having ternary values of a positive value, a negative value, and 0 (zero) is generated according to an input signal. Here, the output signal indicates a difference signal of a voltage PO(+) and a voltage PO(−) which are applied to both ends of a load. When amplitude of the input signal is small, the positive voltage PO(+) and the negative voltage PO(−) become almost equal, and the output signal (PO(+)−PO(−)) becomes nearly 0V, accordingly, almost no current flows through the load.
In driving the load, in order to avoid influence of an induced current resulting from an inductance component of the load, and in order to prevent a high-side power source node and a low-side power source node from short-circuiting, a dead time is provided to set an output switching transistor to OFF state. In the dead time, an output switching transistor in ON state is turned to OFF state, then after the dead time, the following output switching transistor is driven to ON state.
Interdependently affected by the dead time, at the time of inputting a small amplitude signal, a so-called PWM dead zone appears, in which it becomes difficult to generate a PWM signal as a signal to drive an output switching transistor.
Patent Document 1 and Patent Document 2 disclose configuration which aims at improvement of linearity and a frequency response to a command signal, by eliminating such a dead zone due to a dead time.
In Patent Document 1, plural kinds of triangular-wave signals which are offset (level-shifted) in the positive and the negative direction are generated. After determining the polarity of an input signal, one of the plural triangular-wave signals which are offset is chosen and sent to a comparison circuit. The comparison circuit is provided corresponding to each of switching transistors arranged in a full bridge configuration. The comparison circuit compares an input signal with a triangular-wave signal which is individually assigned and offset, and provides a switching control signal based on a command signal to the corresponding transistor according to the comparison result.
By offsetting (level-shifting) triangular-wave signals, Patent Document 1 aims at fully securing a dead time and an “ON” period of a switching control signal and accordingly eliminating the dead zone of a PWM signal, even in a case where the command signal changes in the vicinity of 0V.
In the configuration disclosed by Patent Document 2, a mixed input audio signal is generated by mixing a distortion correcting signal to an input audio signal. Even when the amplitude of the input audio signal is small, it is possible to generate a PWM signal by making a level comparison to a triangular-wave signal with the use of the distortion correcting signal. A PWM signal for switching control is generated with a cycle of the distortion correcting signal at the time of inputting no audio signal. Accordingly, an output switching transistor is turned to ON state intermittently, leading to the suppression of an increase in power consumption.
(Patent Document)    (Patent Document 1) Japanese Patent Laid-open No. 2001-268977    (Patent Document 2) Japanese Patent Laid-open No. 2004-312606